It is known in the art to prepare filtration membranes of gamma alumina by depositing boehmite or bayerite particles from sols onto microporous alumina supports, then drying the deposit. The thickness of the deposited gel can be controlled by control of the time of contact between the sol and the support and the characteristics of the sol, as taught by A. Leenaars et al., 16 Chemtech 560-64 (1986). As described in this reference, the membranes may be used either in dried, but still chemically hydrated form, or they may be heated to give unhydrated gamma alumina ESL's to alpha alumina. However, the pore size rapidly increases as the alumina changes from the gamma or delta phases to alpha, i.e. from about 5 nm to 78 nm, and also the porosity decreases from about 55% to 41%. (page 562)
Gamma alumina is known to have significantly less corrosion resistance than alpha alumina in a variety of practically important environments, such as sodium hypochlorite, sodium hydroxide, and nitric acid solutions in water.
A. Leenaars et al., 19 Journal of Materials Science 1077-1088 (1984) shows in Table I alpha alumina membranes with minimum pore sizes above 38 nm.
S. Wilson et al, 82 Journal of Colloid and Interface Science 507-17 (1981), report alpha aluminas with pore sizes of as little as 25 nm, formed from an intermediate gamma alumina phase with pores about 0.8 nm in size. The products reported by Wilson are believed not to be in membrane form, but rather in the form of powders.
It is also known that the transformation of gelled amorphous alumina hydrates to polycrystalline alpha alumina can be accomplished more rapidly and at lower temperatures than would otherwise be required by seeding or nucleating the deposit with small particles of crystalline alumina. Cottringer et al U.S. Pat. No. 4,623,364 teaches that alpha alumina is effective for this purpose, and J. McArdle et al, 69 Journal of the American Ceramic Society, C98-C101 (1986) teaches that gamma alumina may also be used. These teachings, as well as all previous teachings of nucleated transformation known to the applicant, have been applied to bulk samples in which a goal is the achievement of high density in the product, not the preparation of porous membranes. Indeed, because seeding or nucleation is known to promote densification, as taught by the above references, seeding would be expected to make more difficult the attainment of consistent controlled fine porosity, as required for the preparation of the present ultrafiltration membranes.